userdiff: add Elixir to supported userdiff languages

Adds support for xfuncref in Elixir[1] language which is Ruby-like
language that runs on Erlang[3] Virtual Machine (BEAM).

[1]: https://elixir-lang.org
[2]: https://www.erlang.org

Signed-off-by: Łukasz Niemier <lukasz@niemier.pl>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t4018/elixir-macro b/t/t4018/elixir-macro
new file mode 100644
index 0000000..4f925e9
--- /dev/null
+++ b/t/t4018/elixir-macro
@@ -0,0 +1,5 @@
+defmacro foo(RIGHT) do
+  # Code
+  # Code
+  ChangeMe
+end